There is known a vehicle behavior control system in which when the vehicle gets unstable as a result of the steering wheel being turned drastically, a braking force is given to part of the wheels to stabilize the behavior of the vehicle. For example, in a vehicle behavior control system described in JP-2010-64720-A, an actual turning state quantity such as an actual yaw rate is obtained, and when the vehicle gets unstable, a braking force is given to part of the wheels based on the turning state quantity so obtained, whereby the behavior of the vehicle is stabilized.
More specifically, the vehicle behavior control system of JP-2010-64720-A is configured to restrict an oversteer condition by giving the braking force to turning outer wheels while the vehicle is turning.
In this vehicle behavior control system, in case a disturbance to the behavior of the vehicle can be estimated, it is desirable to start a vehicle behavior stabilization control as soon as possible in order to reduce the disturbance to the behavior of the vehicle as much as possible. For example, in JP-2010-64702-A, a threshold to start a behavior stabilization control is changed in case the steering wheel is turned to an opposite direction after it has been turned either to the left or the right.
Incidentally, it takes, more or less, a certain period of time until a change in vehicle behavior appears after the steering wheel has been operated. Because of this, in case the disturbance to the behavior of the vehicle is estimated based on the actual turning state quantity of the vehicle as in the vehicle behavior control system of JP-2010-64720-A, the control may not be started early enough. In addition, in the vehicle behavior control system of JP-2010-64720-A, the threshold is changed on condition that the steering wheel is turned back, and therefore, even though a first turning of the steering wheel is drastic, the threshold is not changed. As a result, the start of the control may be delayed.
In addition, in the event that the braking force is given to the turning outer wheels of the vehicle to solve the oversteer condition of the vehicle, in case the braking force is too large, the yaw rate is reduced more than required, thereby possibly causing an understeer condition.